Synchronous machines of this type are suitable for operation on a fixed power system, e.g., a fixed three-phase system, as well as for operation via an electronic converter. Moreover, machines of this type are suitable in generator operation for regulating the induced voltage in a multi-phase stator winding system as they are needed in electrical systems of motor vehicles, for example. In these electric machines, the poles on the circumference of the rotor are excited partly by permanent magnets and partly electrically.
Pole-changing synchronous machines are discussed in WO 2004/017496 in which pole-changing of the rotor takes place by changing the direction of the current in the field coils of the rotor, this change being used for controlling the output voltage of a multi-phase stator winding. By continuously regulating the field current, the output voltage of the multi-phase stator winding, which is dimensioned for the higher number of poles, may be influenced in a wide range. For example, three radially magnetized permanent magnets and three field coils are needed in a system which is reversible between twelve and six poles according to FIGS. 1a and 1b, the field coils, having a step size from a pole pitch of the higher number of poles, are each situated on a shank, which is radially oriented to the circumference. The disadvantage here is that, for a symmetrical pole arrangement, one field coil for an appropriate magnetomotive force is needed for each permanent magnet. This requires a correspondingly large copper cross section and takes up plenty of space on the rotor. It is also disadvantageous that the permanent magnets on the rotor circumference have the same polarity which complicates their magnetization in the installed state since the magnetic reflux must take place via adjacent poles.
Additional specific embodiments of synchronous machines of this type are discussed in WO 99/67871. There, however, the permanent magnets in the rotor are positioned in the radial direction and are magnetized in a chord-like manner. Here also, field coils between the permanent magnets, which have a step size from a pole pitch of the higher number of poles, are situated on a shank which runs radially to the circumference. Likewise, the number of field coils is equal to the number of permanent magnets in a symmetrical pole arrangement; all field coils must be designed for the full magnetomotive force. For a machine having a twelve-pole rotor, even four field coils having a correspondingly large copper cross section are needed and require plenty of space. It is disadvantageous here also that complete magnetization of the permanent magnets in the installed state on the rotor is almost impossible because the interior areas of the permanent magnets are not reached to the full extent by a magnetizing head on the rotor circumference.
A multi-phase electric machine having a pole-changing, hybrid-excited rotor is known from US 2007/0090713 A1 which is used in motor vehicles during engine operation for starting the vehicle drive as well as during generator operation for supplying power to the vehicle electrical system. For optimizing this double function, the stator winding of the machine has a number of poles which corresponds to the smaller number of poles of the pole-changing rotor. In addition, the star-connected stator winding is provided with a star point coupling of the downstream bridge rectifier for reducing the machine's electromagnetic losses, whereby the ripple factor of the electromotive force is decreased during generator operation.